Enhancing performing characteristics of organic semiconducting films by improved solution processing

ABSTRACT

Improved processing methods for enhanced properties of conjugated polymer films are disclosed, as well as the enhanced conjugated polymer films produced thereby. Addition of low molecular weight alkyl-containing molecules to solutions used to form conjugated polymer films leads to improved photoconductivity and improvements in other electronic properties. The enhanced conjugated polymer films can be used in a variety of electronic devices, such as solar cells and photodiodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/949,705, filed Dec. 3, 2007, which claims the priority benefit ofU.S. Provisional Patent Application No. 60/872,221, filed Dec. 1, 2006,of U.S. Provisional Patent application No. 60/919,602, filed Mar. 23,2007, and of U.S. Provisional Patent Application No. 60/938,433, filedMay 16, 2007. The entire contents of those applications are herebyincorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made in part during the course of work under grantnumbers DE-FG02-06ER46324 and DE-FG24-04NT42277 from the United StatesDepartment of Energy, grant number N-00014-04-0411 from the Office ofNaval Research of the United States Navy, and under an NDSEG Fellowshipfrom the United States Department of Defense. The United StatesGovernment has certain rights in this invention.

TECHNICAL FIELD

This application relates to polymer-based electronic devices andparticularly to solution processing methods for enhancement of thecharacteristics of organic semiconducting films, such as used in organicphotodetectors, solar cells, and thin film transistors.

BACKGROUND

Solar cells provide a renewable energy source that can be implemented ina wide variety of geographic regions. However, solar cells account foronly a small percentage of energy production (for example, in the UnitedStates, solar energy accounted for 0.065% of energy production in 2005,a figure which includes both photovoltaic and solar thermal energy). Themost common type of photovoltaic cell is made from silicon; however,preparing the high-purity silicon required for their manufacture iscostly both in economic terms and the energy input required to purifythe silicon. Thus silicon-based solar cells are used primarily in remotelocations or in markets where the importance of ecologicalsustainability outweighs the cost of inorganic solar cells.

Organic thin-film-based solar cells, such as polymer-based solar cells,have been the subject of much research as alternatives to the high-costinorganic solar cells. These solar cells are typically fabricated withan electron donor material and an electron acceptor material, whichallows an electron-hole pair (an exciton) generated by a photon toseparate and generate current. The junction between the donor andacceptor can be created by forming a layer of one material (e.g., thedonor) on top of the other material (e.g., the acceptor), which forms aplanar heterojunction between the bilayers. Since the planar bilayerheterojunction affords a relatively small area for charge separation tooccur, different morphologies have been explored. Interpenetratingnetworks of donor and acceptor materials can be used; these range fromdiffuse interfaces at a bilayer heterojunction to bulk heterojunctions,where the donor and acceptor materials are mixed together and form amulti-component active layer.

Bulk heterojunction (BHJ) solar cells can be fabricated from blendscontaining a conjugated polymer and a fullerene derivative and have thepotential to generate inexpensive, flexible, photoconductive devicessuch as photodetectors and solar cells, avoiding the cost constraints ofsilicon-based devices. A major advantage of such plastic solar cellsrests in their ability to be processed from solution, a feature whichmay make polymer-based devices more economically viable thansmall-molecule based organic photovoltaic cells.

DISCLOSURE OF THE INVENTION

The invention relates to methods for modifying organic semiconductorfilms, such as conjugated polymer films, in order to provide improvedperformance characteristics of the films, where the characteristics arephotoconductivity, charge transport, solar conversion efficiency, and/orphotovoltaic efficiency. In one embodiment, the invention embracesmodifying the internal structure or morphology of organic semiconductorfilms in order to provide improved performance characteristics. In oneembodiment, the conjugated polymer films are bulk heterojunction (BHJ)films, and the invention relates to methods for improved or increasedphotoconductivity, charge transport, solar conversion efficiency, and/orphotovoltaic efficiency of the BHJ films, or devices fabricated from BHJfilms. In another embodiment, the invention embraces films havingimproved or increased photoconductivity, charge transport, solarconversion efficiency, and/or photovoltaic efficiency. The inventionalso relates to electronic devices incorporating organic semiconductorfilms, conjugated polymer films, and/or bulk heterojunction filmsprocessed according to the methods disclosed herein.

In another embodiment, the invention embraces methods of modifyingorganic semiconductor films, conjugated polymer films, and/or bulkheterojunction films by adding a processing additive to a solution usedto form the film, prior to the formation of the film.

In one embodiment, the invention embraces a method of increasing thephotoconductivity, charge transport, solar energy conversion efficiency,or photovoltaic efficiency of an organic semiconductor film, comprisingthe steps of adding an amount of one or more low molecular weightalkyl-containing molecules to a solution of one or more organicsemiconductors, and forming the organic semiconductor film from thesolution.

In one embodiment, the one or more low molecular weight alkyl-containingmolecules are selected from alkanes, alcohols, and alkyl thiols. Inanother embodiment, the low molecular weight alkyl-containing moleculesare selected from C₁-C₂₀ alkanes. In another embodiment, the lowmolecular weight alkyl-containing molecules are selected from C₁-C₂₀alcohols. In another embodiment, the low molecular weightalkyl-containing molecules are selected from C₁-C₂₀ alkanethiols.

In one embodiment, the one or more low molecular weight alkyl-containingmolecules is present in the solution used to form the conjugated polymerfilm in an amount of about 0.1% to about 10% v/v.

The organic semiconductor film can comprise a conjugated polymer film.The conjugated polymer film can serve as an electron donor;alternatively, the conjugated polymer film can serve as an electronacceptor. When the conjugated polymer film serves as an electron donor,the organic semiconductor film can additionally comprise an organicelectron acceptor; when the conjugated polymer film serves as anelectron acceptor, the organic semiconductor film can additionallycomprise an organic electron donor. The organic electron donor and/orthe organic electron acceptor can be a second conjugated polymer film.

In one embodiment, the conjugated polymer film electron donor comprisesa polymer selected from polyacetylene, a polyphenylene,poly(3-alkylthiophenes) where alkyl is from 6 to 16 carbons (P3AT's),poly-(3-hexylthiophene) (P3HT),poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT), polyphenylacetylene, polydiphenylacetylene, polyanilines,poly(p-phenylene vinylene) (PPV) and alkoxy derivatives thereof,poly(2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV),poly(2,5-dimethoxy-p-phenylene vinylene) (PDMPV), a polythiophene, apoly(thienylenevinylene), poly(2,5-thienylenevinylene), a polyporphyrin,a porphyrinic macrocycle, a thiol-derivatized polyporphyrin, apolymetallocene, a polyferrocene, a polyphthalocyanine, a polyvinylene,a polyphenylvinylene, a polysilane, a polyisothianaphthalene, or apolythienylvinylene. In another embodiment, the conjugated polymer filmelectron donor can comprise a derivative of one or more of the foregoingmaterials. In another embodiment, the conjugated polymer film electrondonor can comprise a blend or combination of two or more of theforegoing materials in any proportion.

The organic electron acceptor can be a fullerene derivative, such as afullerene derivative selected from compounds of the formula:

where the circle indicated as “fullerene” is independently selected froma C₆₀, C₇₀, or C₈₄ fullerene moiety; Ar is independently phenyl orthienyl, which can be optionally substituted; R₁ is independently C₁-C₁₂alkyl; and R₂ is independently —O—C₁-C₁₂ alkyl or —O—C₁-C₁₂ alkyl-SH. Inone embodiment, the fullerene derivative is C61-PCBM. In anotherembodiment, the fullerene derivative is C71-PCBM.

In one embodiment where the organic semiconductor film comprises aconjugated polymer electron donor/fullerene derivative electronacceptor, the ratio of polymer to fullerene derivative can range fromabout 5:1 to 1:10, about 2:1 to 1:5, preferably about 1:1 to 1:5, morepreferably about 1:2 to 1:4 or about 1:2 to 1:3. In other embodiments,the polymer concentration can be about 0.01% to 10% by weight of thesolution, about 0.1% to 10% by weight of the solution, about 0.1% to 5%by weight of the solution, about 0.5% to 5% by weight of the solution,about 0.5% to 3% by weight of the solution, about 0.5% to 3% by weightof the solution, about 0.5% to 2% by weight of the solution, about 0.5%to 1% by weight of the solution, or about 0.8% to 1% by weight of thesolution.

The organic semiconductor film can be fabricated in any form thatprovides for separation of electron-hole pairs. In one embodiment, theorganic semiconductor film is fabricated from a conjugated polymer filmelectron donor and an organic electron acceptor in a planar bilayerform. In another embodiment, the organic semiconductor film isfabricated from a conjugated polymer film electron donor and an organicelectron acceptor in a bilayer form with a diffuse interface. In anotherembodiment, the organic semiconductor film is fabricated from aconjugated polymer film electron donor and an organic electron acceptorin a bulk heterojunction form.

The organic semiconductor film in any of its forms, or the conjugatedpolymer film, can be formed by spin casting, doctor blading,drop-casting, sequential spin-casting, formation of Langmuir-Blodgettfilms, electrostatic adsorption techniques, and/or dipping the substrateinto the solution. Subsequent processing steps can include evaporationof the solvent to form the film, optionally under reduced pressureand/or elevated temperature; and thermal annealing of the depositedfilm. In one embodiment, when the film is formed by spin casting, thespin speed can range from about 500 to 2000 RPM, or about 1200 RPM to1600 RPM.

The solvent used to form the organic semiconductor film can be selectedfrom chlorobenzene, dichlorobenzene, trichlorobenzene, benzene, toluene,chloroform, dichloromethane, dichloroethane, xylenes,α,α,α-trichlorotoluene, methyl naphthalene, chloronaphthalene, ormixtures thereof. Dichlorobenzene can include o-dichlorobenzene,m-dichlorobenzene, p-dichlorobenzene, or mixtures thereof in anyproportion. Methyl naphthalene can include 1-methylnaphthalene,2-methylnaphthalene, or mixtures thereof in any proportion.Chloronaphthalene can include 1-chloronaphthalene, 2-chloronaphthalene,or mixtures thereof in any proportion.

In one embodiment, the invention embraces organic semiconductor filmsfabricated according to the methods of the invention. In anotherembodiment, the invention embraces electronic devices formed fromorganic semiconductor films fabricated according to the methods of theinvention; these devices include, but are not limited to, solar cells,photovoltaic cells, photodetectors, photodiodes, or phototransistors.Electronic devices according to the invention can comprise a firstelectrode, an organic semiconductor film having a first side and asecond side, where the first side of the organic semiconductor filmcontacts the first electrode and where the film is formed by adding anamount of one or more low molecular weight alkyl-containing molecules toa solution used to form the organic semiconductor film, and a secondelectrode contacting the second side of the organic semiconductor film.The organic semiconductor film of the devices can comprise a conjugatedpolymer film. The first electrode and the second electrode can havedifferent work functions. In one embodiment, the first electrode can bea high work function material. The second electrode can be a low workfunction material.

In another embodiment, the invention embraces a method of increasing thephotoconductivity, charge transport, solar energy conversion efficiency,or photovoltaic efficiency of an organic semiconductor film, comprisingthe steps of adding an amount of one or more low molecular weightalkyl-containing molecules to a solution of one or more organicsemiconductors, and forming the organic semiconductor film from thesolution, wherein the low molecular weight alkyl-containing moleculesare selected from the group consisting of C₁-C₂₀ alkanes substitutedwith one or more substituents selected from aldehyde, dioxo, hydroxy,alkoxy, thiol, thioalkyl, carboxylic acid, ester, amine, amide, ether,thioether, halide, fluoride, chloride, bromide, iodide, nitrile,epoxide, aromatic, and arylalkyl groups, with the proviso that if athiol or hydroxy group substituent is present, at least oneindependently chosen additional substituent must also be present, andwith the proviso that di-halo substituted compounds are excluded fromthe low molecular weight alkyl-containing molecules. In anotherembodiment, poly-halo substituted compounds are excluded from the lowmolecular weight alkyl-containing molecules. In another embodiment,mono-halo substituted compounds are excluded from the low molecularweight alkyl-containing molecules.

In one embodiment, the low molecular weight alkyl-containing moleculesare selected from C₁-C₂₀ alkanes substituted with at least one hydroxygroup and at least one thiol group. In another embodiment, the one ormore low molecular weight alkyl-containing molecules are selected fromC₁-C₂₀ dithioalkanes; the C₁-C₂₀ dithioalkanes can be alpha,omega-substituted. In another embodiment, the low molecular weightalkyl-containing molecules are selected from C₁-C₂₀ iodoalkanes.

In one embodiment, the one or more low molecular weight alkyl-containingmolecules is present in the solution used to form the conjugated polymerfilm in an amount of about 0.1% to about 10% v/v.

The organic semiconductor film can comprise a conjugated polymer film.The conjugated polymer film can serve as an electron donor;alternatively, the conjugated polymer film can serve as an electronacceptor. When the conjugated polymer film serves as an electron donor,the organic semiconductor film can additionally comprise an organicelectron acceptor; when the conjugated polymer film serves as anelectron acceptor, the organic semiconductor film can additionallycomprise an organic electron donor. The organic electron donor and/orthe organic electron acceptor can be a second conjugated polymer film.

In one embodiment, the conjugated polymer film electron donor comprisesa polymer selected from polyacetylene, a polyphenylene,poly(3-alkylthiophenes) where alkyl is from 6 to 16 carbons (P3AT's),poly-(3-hexylthiophene) (P3HT),poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT), polyphenylacetylene, polydiphenylacetylene, polyanilines,poly(p-phenylene vinylene) (PPV) and alkoxy derivatives thereof,poly(2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV),poly(2,5-dimethoxy-p-phenylene vinylene) (PDMPV), a polythiophene, apoly(thienylenevinylene), poly(2,5-thienylenevinylene), a polyporphyrin,a porphyrinic macrocycle, a thiol-derivatized polyporphyrin, apolymetallocene, a polyferrocene, a polyphthalocyanine, a polyvinylene,a polyphenylvinylene, a polysilane, a polyisothianaphthalene, or apolythienylvinylene. In another embodiment, the conjugated polymer filmelectron donor can comprise a derivative of one or more of the foregoingmaterials. In another embodiment, the conjugated polymer film electrondonor can comprise a blend or combination of two or more of theforegoing materials in any proportion.

The organic electron acceptor can be a fullerene derivative, such as afullerene derivative selected from compounds of the formula:

where the circle indicated as “fullerene” is independently selected froma C₆₀, C₇₀, or C₈₄ fullerene moiety; Ar is independently phenyl orthienyl, which can be optionally substituted; R₁ is independently C₁-C₁₂alkyl; and R₂ is independently —O—C₁-C₁₂ alkyl or —O—C₁-C₁₂ alkyl-SH. Inone embodiment, the fullerene derivative is C61-PCBM. In anotherembodiment, the fullerene derivative is C71-PCBM.

In one embodiment where the organic semiconductor film comprises aconjugated polymer electron donor/fullerene derivative electronacceptor, the ratio of polymer to fullerene derivative can range fromabout 5:1 to 1:10, about 2:1 to 1:5, preferably about 1:1 to 1:5, morepreferably about 1:2 to 1:4 or about 1:2 to 1:3. In other embodiments,the polymer concentration can be about 0.01% to 10% by weight of thesolution, about 0.1% to 10% by weight of the solution, about 0.1% to 5%by weight of the solution, about 0.5% to 5% by weight of the solution,about 0.5% to 3% by weight of the solution, about 0.5% to 3% by weightof the solution, about 0.5% to 2% by weight of the solution, about 0.5%to 1% by weight of the solution, or about 0.8% to 1% by weight of thesolution.

The organic semiconductor film can be fabricated in any form thatprovides for separation of electron-hole pairs. In one embodiment, theorganic semiconductor film is fabricated from a conjugated polymer filmelectron donor and an organic electron acceptor in a planar bilayerform. In another embodiment, the organic semiconductor film isfabricated from a conjugated polymer film electron donor and an organicelectron acceptor in a bilayer form with a diffuse interface. In anotherembodiment, the organic semiconductor film is fabricated from aconjugated polymer film electron donor and an organic electron acceptorin a bulk heterojunction form.

The organic semiconductor film in any of its forms, or the conjugatedpolymer film, can be formed by spin casting, doctor blading,drop-casting, sequential spin-casting, formation of Langmuir-Blodgettfilms, electrostatic adsorption techniques, and/or dipping the substrateinto the solution. Subsequent processing steps can include evaporationof the solvent to form the film, optionally under reduced pressureand/or elevated temperature; and thermal annealing of the depositedfilm. In one embodiment, when the film is formed by spin casting, thespin speed can range from about 500 to 2000 RPM, or about 1200 RPM to1600 RPM.

The solvent used to form the organic semiconductor film can be selectedfrom chlorobenzene, dichlorobenzene, trichlorobenzene, benzene, toluene,chloroform, dichloromethane, dichloroethane, xylenes,α,α,α-trichlorotoluene, methyl naphthalene, chloronaphthalene, ormixtures thereof. Dichlorobenzene can include o-dichlorobenzene,m-dichlorobenzene, p-dichlorobenzene, or mixtures thereof in anyproportion. Methyl naphthalene can include 1-methylnaphthalene,2-methylnaphthalene, or mixtures thereof in any proportion.Chloronaphthalene can include 1-chloronaphthalene, 2-chloronaphthalene,or mixtures thereof in any proportion.

In one embodiment, the invention embraces organic semiconductor filmsfabricated according to the methods of the invention. In anotherembodiment, the invention embraces electronic devices formed fromorganic semiconductor films fabricated according to the methods of theinvention; these devices include, but are not limited to, solar cells,photovoltaic cells, photodetectors, photodiodes, or phototransistors.Electronic devices according to the invention can comprise a firstelectrode, an organic semiconductor film having a first side and asecond side, where the first side of the organic semiconductor filmcontacts the first electrode and where the film is formed by adding anamount of one or more low molecular weight alkyl-containing molecules toa solution used to form the organic semiconductor film, and a secondelectrode contacting the second side of the organic semiconductor film,wherein the low molecular weight alkyl-containing molecules are selectedfrom the group consisting of C₁-C₂₀ alkanes substituted with one or moresubstituents selected from aldehyde, dioxo, hydroxy, alkoxy, thiol,thioalkyl, carboxylic acid, ester, amine, amide, ether, thioether,halide, fluoride, chloride, bromide, iodide, nitrile, epoxide, aromatic,and arylalkyl groups, with the proviso that if a thiol or hydroxy groupsubstituent is present, at least one independently chosen additionalsubstituent must also be present. The organic semiconductor film of thedevices can comprise a conjugated polymer film. The first electrode andthe second electrode can have different work functions. In oneembodiment, the first electrode can be a high work function material.The second electrode can be a low work function material.

In one embodiment of the invention, di-halo substituted compounds areexcluded from the low molecular weight alkyl-containing molecules. Inanother embodiment, poly-halo substituted compounds are excluded fromthe low molecular weight alkyl-containing molecules. In anotherembodiment, mono-halo substituted compounds are excluded from the lowmolecular weight alkyl-containing molecules. In another embodiment, anyhalo-substituted compound is excluded from the low molecular weightalkyl-containing compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts transmittance through films, for films on glasssubstrates: BHJ-NA (x's, X), BHJ-A (asterisks or *,

), BHJ-1% C8-NA (filled diamonds, ♦), and BHJ-1% C8-A (open diamonds,⋄).

FIG. 2 depicts the effect on photoconductivity of (FIG. 2 a) annealingand enrichment with 5% n-octanethiol; (FIG. 2 b) alkyl thiol chainlength; and (FIG. 2 c) n-octanethiol concentration. Samples are onalumina substrates and consist of: BHJ-NA indicated by x's (X), BHJ-Aindicated by asterisks * ( )

, BHJ-5% C8-A indicated by open circles (◯), BHJ-5% C8-NA indicated byfilled circles (●), BHJ-5% C12-A indicated by filled squares (▪), BHJ-5%C6-A by open squares (□), BHJ-0.1% C8-A by open inverted triangles (∇),BHJ-0.75% C8-NA by filled diamonds (♦), BHJ-0.75% C8-A by open diamonds(⋄), BHJ-1% C8-A by open triangles (Δ), and BHJ-10% C8-A by filledtriangles (▴).

FIG. 3 depicts a comparison of device efficiencies for the BHJ-C8-NAseries (solid squares (▪) and solid lines), and the BHJ-C8-A series(open squares (□) and dashed lines). The flat lines are the efficienciesobtained using BHJ-NA and BHJ-A. (C8 indicates 1-octanethiol.)

FIG. 4 depicts transistor behavior of films in log scale: BHJ-NA (dashedline) and BHJ-5% C8-NA (solid line) at gate voltages of 0, −7.5, −15,−22.5, and −30 Volts.

FIG. 5 depicts X-ray diffraction results highlighting the <100>diffraction peak for P3HT in P3HT/PCBM films cast from Toluene BHJ-NA(x's, X), BHJ-A (asterisks or *,

), BHJ-0.75% C8-NA (filled diamonds, ♦), and BHJ-0.75% C8-A (opendiamonds, ⋄).

FIG. 6 depicts normalized transient photocurrent waveforms (at the peak)of films: BHJ-NA (dotted line) and BHJ-1% C8-NA (solid line).

FIG. 7 depicts the effect of dodecane thiol additive (0% or control, 1%,1.5% v/v) on the power conversion efficiency of solar cells fabricatedfrom PCPDTBT and Phenyl-C60 butyric acid methyl ester (14:14 mg/ml) castfrom chlorobenzene.

FIG. 8 depicts the effect of dodecane thiol additive (0% or control,0.5%, 1%, 2% v/v) on the power conversion efficiency of solar cellsfabricated from PCPDTBT and Phenyl-C70 butyric acid methyl ester (10:20mg/ml) cast from chlorobenzene.

FIG. 9 depicts the effect of various additives at 1% (butanethiol,octanethiol, 1,8-octanedithiol, heptyl cyanide, 1,8-dicyanooctane,1,8-dimethyl suberate), as well as for solvent without additive, on thepower conversion efficiency of solar cells fabricated from PCPDTBT andPhenyl-C70 butyric acid methyl ester cast from chlorobenzene.

FIG. 10 depicts the effect of various dithiol additives(1,4-butanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol,1,9-nonanedithiol), as well as for solvent without additive, on thepower conversion efficiency of solar cells fabricated from PCPDTBT andPhenyl-C70 butyric acid methyl ester cast from chlorobenzene.

FIG. 11 depicts the effect of different spin speeds for spin-casting of2% dithiol solutions, as well as for solvent without additive, on thepower conversion efficiency of solar cells fabricated from PCPDTBT andPhenyl-C70 butyric acid methyl ester cast from chloroform.

FIG. 12 depicts the effect of different fullerene acceptors on the powerconversion efficiency of solar cells fabricated using PCPDTBT donor castfrom chlorobenzene with and without 2% octanedithiol.

FIG. 13 depicts the effect of 2% 1,8-octanedithiol on the powerconversion efficiency of solar cells fabricated from PCPDTBT andPhenyl-C70 butyric acid methyl ester cast from xylene, as well as forsolvent without additive.

FIG. 14 depicts the effect of different additives (hexanethiol andoctanedithiol), as well as for solvent without additive, on the powerconversion efficiency of solar cells fabricated from P3HT and Phenyl-C60butyric acid methyl ester cast from toluene before and after annealing.

FIG. 15 depicts the effect of different additives (hexanethiol andoctanol), as well as for solvent without additive, on the powerconversion efficiency of solar cells fabricated from P3HT and Phenyl-C60butyric acid methyl ester cast from toluene before and after annealing.

FIG. 16 depicts the effect of different additives (dodecanethiol andoctanethiol), as well as for solvent without additive, on the powerconversion efficiency of solar cells fabricated from P3HT and Phenyl-C60butyric acid methyl ester cast from toluene after annealing.

FIG. 17 depicts the effect of different additives (octanethiol andundecane), as well as for solvent without additive, on the powerconversion efficiency of solar cells fabricated from P3HT and Phenyl-C60butyric acid methyl ester cast from toluene before and after annealing.

FIG. 18 depicts the effect of different additives (octanethiol andundecane), as well as for solvent without additive, on the powerconversion efficiency of solar cells fabricated from P3HT and Phenyl-C60butyric acid methyl ester cast from toluene before and after annealingwherein undecane has a positive effect on device performance.

FIG. 19 depicts the effect of different additives (octanethiol andbutanethiol), as well as for solvent without additive, on the powerconversion efficiency of solar cells fabricated from P3HT and Phenyl-C60butyric acid methyl ester cast from chlorobenzene before and afterannealing.

FIG. 20 depicts the effect of different additives (octanol, octane, andoctanethiol, each at 1%; solvent only, without additive, is displayedfor comparison at left) on the power conversion efficiency of solarcells fabricated from P3HT and Phenyl-C60 butyric acid methyl ester castfrom chlorobenzene before and after annealing wherein octanol has apositive effect on device performance.

FIG. 21 depicts the effect of dodecanethiol, at 1%, 2%, and 3% (solventonly, without additive, is displayed for comparison at left), on thepower conversion efficiency of solar cells fabricated from P3HT andPhenyl-C60 butyric acid methyl ester cast from chloroform before andafter annealing.

FIG. 22 depicts the effect of phenylhexane, at 1%, 2%, and 3% (solventonly, without additive, is displayed for comparison at left), on thepower conversion efficiency of solar cells fabricated from P3HT andPhenyl-C60 butyric acid methyl ester cast from chlorobenzene before andafter annealing.

FIG. 23 depicts the effect of dodecanethiol, at 1%, 2%, and 3% (solventonly, without additive, is displayed for comparison at left), on thepower conversion efficiency of solar cells fabricated from P3HT andPhenyl-C60 butyric acid methyl ester cast from 1,2 dichlorobenzenebefore and after annealing.

FIG. 24 depicts the effect of different additives (octanethiol andoctanedithiol, each at 1%; solvent only, without additive, is displayedfor comparison at left) on the power conversion efficiency of solarcells fabricated from either P3HT orpoly[3-(ethyl-4-butanoate)thiophene-2,5-diyl] and Phenyl-C60 butyricacid methyl ester cast from chlorobenzene before and after annealing.

FIG. 25 depicts the effect of different additives (octanethiol andoctanedithiol, each at 1%; solvent only, without additive, is displayedfor comparison at left) on the power conversion efficiency of solarcells fabricated from P3HT and either Phenyl-C60 butyric acid methylester or Phenyl-C60 butyric acid dodecyl ester cast from chlorobenzenebefore and after annealing.

FIG. 26 depicts the effect of 2% octanedithiol (compared to no additive)on the power conversion efficiency of solar cells fabricated from P3HTand Phenyl-C70 butyric acid methyl ester cast from chlorobenzene beforeand after annealing.

FIG. 27 depicts the change in power conversion efficiency versusoctanethiol concentration for P3HT:C60-PCBM films cast from toluene at120 nm (squares) and 220 nm (circles) before annealing (closed shapes)and after annealing (open shapes).

FIG. 28 depicts current-voltage characteristics of solar cells withP3HT/PCBM cast from CB (squares) and CB/2.5% octanethiol (circles)before (solid) and after thermal annealing (open).

FIG. 29 depicts the effect of 1% phenyl hexane (thin lines) andannealing (dotted lines are annealed) on the external quantum efficiencyspectra of P3HT:C60-PCBM devices.

FIG. 30 depicts X-ray diffraction results highlighting the <100>diffraction peak for P3HT in P3HT/C-60 PCBM films cast fromchlorobenzene (squares) or chlorobenzene containing 2.5% octanethiol(circles) either before annealing (closed shapes) or after annealing(open shapes).

FIG. 31 depicts increased charge carrier mobility in films afterprocessing with 1% thiol additive. From left to right: Au, no additive,holes; Au, octanethiol, holes; Al, no additive, holes; Al,dodecanethiol, holes; Al, no additive, electrons; Al, dodecanethiol,electrons.

FIG. 32 depicts peak absorption changes before (black bars) and after(shaded bars) annealing for films cast with various additives (at 1%)relative to control. From left to right, the pairs of bars are: control;1-octanethiol; 1,8-octanedithiol; 1-octanol; methyl caprylate; dimethylsuberate; heptyl cyanide; 1,6-dicyanohexane.

FIG. 33 depicts absorption changes at 600 nm before (black bars) andafter (shaded bars) annealing for films cast with various additives (at1%) relative to control. From left to right, the pairs of bars are:control; 1-octanethiol; 1,8-octanedithiol; 1-octanol; methyl caprylate;dimethyl suberate; heptyl cyanide; 1,6-dicyanohexane.

FIG. 34 depicts steady state photoconductivity of P3HT:C60-PCBM(circles) and PCPDTBT:C70-PCBM (squares) processed from chlorobenzene(solid shapes) and with 1,8-octanedithiol (open shapes).

FIG. 35 depicts UV-VIS absorption spectra of PCPDTBT:C₇₁-PCBM films castusing different solution additives. The spectral response indicates ared-shift from a pure PCPDTBT:C₇₁-PCBM film (squares) to films cast fromchlorobenzene containing 2.4 mg/mL 1,3-propanedithiol (diamonds),1,4-butanedithiol (triangles), 1,6-hexanedithiol (stars) and1,8-octanedithiol (circles).

FIG. 36 depicts device IV characteristics, as current density vs.voltage curves (under simulated AM1.5 G radiation at 80 mW/cm²) for aseries of PCPDTBT:C₇₁-PCBM solar cells. The PCPDTBT:C₇₁-PCBM films werecast at 1200 RPM from chlorobenzene with no additive (squares) andchlorobenzene containing 2.4 mg/mL butanedithiol (triangles),hexanedithiol (stars), or octanedithiol (circles).

FIG. 37 depicts the incident photon conversion efficiency of varioussolar cells. The top panel (FIG. 37A) depicts IPCE spectra of polymerbulk heterojunction solar cells composed of P3HT:C₆₁-PCBM before (solidsquares) and after (open squares) annealing, and PCPDTBT:C₇₁-PCBM with(open circles) and without (closed circles) the use of1,8-octanedithiol. The AM 1.5 global reference spectrum is shown forreference (line). The bottom panel (FIG. 37B) depicts current voltagecharacteristics of the same PCPDTBT devices used for the IPCEmeasurements, processed with (solid) and without (dotted line)1,8-octanedithiol under simulated 100 mW/cm² AM 1.5 G illumination;I_(sc)=16.2 mA/cm², FF=0.55, and V_(oc)=0.62 V.

MODES FOR CARRYING OUT THE INVENTION

Various embodiments of the present invention are described hereinafter.It should be noted that an aspect described in conjunction with aparticular embodiment of the present invention is not necessarilylimited to that embodiment and can be practiced in any other embodimentsof the present invention. For instance, in the following description,some embodiments of the present invention are described with embodimentsof polymer blends such as P3HT/C₆₁-PCBM, and PCPDTBT-C₇₁-PCBM. It willbe appreciated that the claimed invention may also be used with otherpolymers blends.

High charge separation efficiency combined with the reduced fabricationcosts associated with solution processing and the potential forimplementation on flexible substrates highlight the promising potentialof “plastic” solar cells. Attempts to control the donor/acceptormorphology in bulk heterojunction materials as required for achievinghigh power conversion efficiency have, however, met with limitedsuccess. By incorporating additives such as alkanedithiols in thesolution used to spin cast films comprising a low bandgap polymer and afullerene derivative, the power conversion efficiency of photovoltaiccells (AM 1.5 G conditions) is increased; e.g., with an alkanedithiol,power conversion efficiency is increased from 2.8% to 5.5% throughaltering the bulk heterojunction morphology.

Some embodiments of the invention embrace improved bulk heterojunction“plastic” solar cells, which are based on phase—separated blends ofpolymer semiconductors and fullerene derivatives. Because ofself-assembly on the nanometer length-scale, mobile carriers andexcitons formed after absorption of solar irradiation diffuse to aheterojunction prior to recombination and are dissociated at thepolymer/fullerene interface. Ultrafast charge transfer fromsemiconducting polymers to fullerenes ensures that the quantumefficiency for charge transfer (CT) at the interface approaches unity,with electrons on the fullerene network and holes on the polymernetwork. After breaking the symmetry by using different metals for thetwo electrodes, electrons migrate toward the lower work function metaland holes migrate toward the higher work function metal. Despite highcharge separation efficiency, a significant fraction of carriersrecombine at donor/acceptor interfaces prior to extraction from thedevice due, in part, to the inherently random interpenetrating networkmorphology formed after spin casting. Carrier recombination prior toreaching the electrodes and low mobility limit both the device fillfactor (FF) and the overall photon harvesting by reducing the optimumactive layer thickness. The carrier lifetime is largely controlled bythe phase morphology between the donor and acceptor materials. Althoughsignificant advancements in the performance of polymer-basedphotovoltaic devices have been made during the past few years, theability to control the morphology of the donor/acceptor network iscritical to optimizing efficiency.

It has been discovered that adding certain compounds to the solutionsfrom which the organic semiconductor films, such as bulk heterojunctionfilms, are cast can modify the phase separation and phase morphology ofthe films. This approach offers the potential to introduce morphologycontrol to bulk heterojunction materials during device fabricationwithout need for subsequent thermal annealing.

The most efficient bulk heterojunction devices to date have utilized thehigh mobility semiconducting polymer, P3HT, as the photo-donor, and thesoluble fullerene derivative, [6,6]-phenyl C₆₁-butyric acid methyl ester(C₆₁-PCBM), as the acceptor. Despite considerable effort, however, powerconversion efficiencies obtained from P3HT:C₆₁-PCBM solar cells arelimited to values of approximately 5% due principally to the pooroverlap between the P3HT absorption spectrum and the solar emissionspectrum. With improved light harvesting in the near infra-red (NIR) bya low bandgap polymer, such aspoly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT), higher power conversion efficiencies should be attainable.The energy gap of PCPDTBT is nearly ideal for solar photovoltaicapplications, E_(g)=1.46 eV. The molecular structure of PCPDTBT is

An asymmetric C₇₀ fullerene was chosen for some embodiments because ofits increased absorption in the visible, which leads to better overlapwith the solar spectrum relative to that obtained with the C₆₁ analogue.Thermal annealing or controlled solvent evaporation after film castinghave proven desirable for optimizing charge separation and transportwithin the bulk heterojunction morphology; unfortunately, attempts toimprove the performance of PCPDTBT:C₇₁-PCBM solar cells through similarmethods have not been successful. The present invention providesstraightforward and cost-effective methods of improving performance ofsuch cells.Alkyl-Containing Molecules

A variety of alkyl-containing molecules can be used in the instantinvention. An “alkyl-containing molecule” is defined as a molecule thatcontains at least one sp³-hybridized carbon, where the at least onesp³-hybridized carbon is bonded to at least two hydrogen atoms. Thus,molecules containing —CH₂— or —CH₃ fall within the definition ofalkyl-containing molecules, as does methane (CH₄). A low molecularweight alkyl-containing molecule is an alkyl-containing molecule ofmolecular weight equal to or less than about 1000 daltons. Thealkyl-containing molecule must also be able to be included in thesolution used to form or deposit the polymer film. If thealkyl-containing molecule is a liquid, it can be mixed into thesolution; if it is a gas, it can be bubbled or otherwise dispersed intothe solution; if it is a solid, it can be dissolved or melted into thesolution or included as a suspension. Finally, the alkyl-containingmolecules should be sufficiently stable to be suitable for thedeposition process and unreactive with the materials in the organicsemiconductor layer and other components of devices which they maycontact.

Alkyl-containing molecules can be substituted with a functional groupselected from aldehyde (—C(═O)—H), dioxo (i.e., an sp²-hybridized oxygenmolecule occupying two valences on a carbon atom to form a ketone,—C(═O)—), hydroxy (—OH), alkoxy (—O—C₁-C₁₂ alkyl), thiol (—SH),thioalkyl (—S—C₁-C₁₂ alkyl), carboxylic acid (—COOH), ester(—C(—O)—O—C₁-C₁₂ alkyl), amine (—NH₂, —NH(C₁-C₁₂ alkyl), or —N(C₁-C₁₂alkyl)₂), amide (—C(═O)—NH₂, —C(═O)—NH(C₁-C₁₂ alkyl), or —C(═O)—N(C₁-C₁₂alkyl)₂), ether (—O—), thioether (sulfide) (—S—), halide (such asfluoride, chloride, bromide, and iodide)), nitrile (—CN), epoxide,aromatic groups such as C₆-C₁₀ aryl, and arylalkyl groups. Aromaticgroups include, but are not limited to, C₆-C₁₀ aryl groups, such asphenyl, benzyl, and naphthyl groups. Arylalkyl (or aralkyl) groupsinclude, but are not limited to, C₀-C₆alkyl-C₆-C₁₀ aryl-C₀-C₆ alkylgroups.

Examples of aldehydes include, but are not limited to, C₁-C₁₂alkyl-C(═O)—H; examples of ketones include, but are not limited to,C₁-C₁₂ ketones, examples of hydroxy—substituted compounds (alcohols)include, but are not limited to, C₁-C₁₂ alkyl-OH; examples of alkoxycompounds include, but are not limited to, C₁-C₁₂ alkyl substituted with—O—C₁-C₁₂ alkyl; examples of thiol compounds include, but are notlimited to, C₁-C₁₂ alkyl-SH; examples of thioalkyl compounds include,but are not limited to, C₁-C₁₂ alkyl substitued with —S—C₁-C₁₂ alkyl;examples of carboxylic acid compounds include, but are not limited to,C₁-C₁₂ alkyl-COOH; examples of ester compounds include, but are notlimited to, C₁-C₁₂ alkyl-C(═O)—O—C₁-C₁₂ alkyl; examples of aminecompounds include, but are not limited to, C₁-C₁₂ alkyl-NH₂, C₁-C₁₂alkyl-NH(C₁-C₁₂ alkyl), or C₁-C₁₂ alkyl-N(C₁-C₁₂ alkyl)₂; examples ofamide compounds include, but are not limited to, C₁-C₁₂ alkyl-C(═O)—NH₂,C₁-C₁₂ alkyl-C(═O)—NH(C₁-C₁₂ alkyl), or C₁-C₁₂ alkyl-C(═O)—N(C₁-C₁₂alkyl)₂); examples of ether compounds include, but are not limited to,C₁-C₁₂ alkyl-O—C₁-C₁₂ alkyl; examples of thioether compounds include,but are not limited to, but are not limited to, C₁-C₁₂ alkyl-S—C₁-C₁₂alkyl; examples of halide compounds include, but are not limited to,C₁-C₁₂ alkyl-X (where X is F, Cl, Br, or I); examples of nitrilecompounds include, but are not limited to, C₁-C₁₂ alkyl-CN; examples ofepoxide compounds include, but are not limited to, C₁-C₁₂ alkyl wheretwo adjacent carbon atoms are bridged by an oxygen atom to form athree-membered epoxide ring, and also include, but are not limited to,C₁-C₁₂ alkyl-(C₂H₃O); examples of aromatic-substituted alkyl compoundsinclude, but are not limited to, C₁-C₁₂ alkyl-C₆-C₁₀ aryl; examples ofarylalkyl-substituted alkyl compounds include, but are not limited to,C₁-C₁₂ alkyl-C₆-C₁₀ aryl-C₀-C₁₂ alkyl. All groups on all compounds canbe chosen independently, e.g., for the amide compound C₁-C₁₂alkyl-C(═O)—N(C₁-C₁₂ alkyl)₂, the three C₁-C₁₂ alkyl groups can beindependently chosen, such as the compoundCH₃CH₂CH₂CH₂—(C═O)—N(CH₃)(CH₂CH₃). The substituent can be attached toany carbon of the alkyl it substitutes; for example, C₃ alkyl-OH canindicate 1-propanol (HO—CH₂CH₂CH₃) or 2-propanol (CH₃CH(OH)CH₃).

In one embodiment, the alkyl-containing molecules are selected fromalkanes, alcohols, and thiols.

Alkanes include, but are not limited to, C₁-C₂₀ alkanes, such asmethane, ethane, propane, butane, pentane, hexane, heptane, octane,nonane, decane, undecane, dodecane, tridecane, tetradecane, pentadecane,hexadecane, octadecane, nonadecane, and eicosane (icosane). Alkyls canbe straight-chain (n-alkyl), branched-chain, or cyclic; cyclic alkanescan be substituted with n-alkyl and/or branched alkyl groups. In anotherembodiment, the alkanes are selected from C₆-C₁₂ alkanes, such ashexane, heptane, octane, nonane, decane, undecane, and dodecane. Inother embodiments, the alkanes are selected from C₄-C₂₀ alkanes, C₄-C₁₆alkanes, C₅-C₂₀ alkanes, or C₅-C₁₆ alkanes. In other embodiments, thealkanes are selected from C₁-C₂₀ n-alkanes (methane, ethane, propane,n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane,n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane,n-hexadecane, n-octadecane, n-nonadecane, and n-eicosane), C₄-C₂₀n-alkanes, C₄-C₁₆ n-alkanes, C₅-C₂₀ n-alkanes, C₅-C₁₆ n-alkanes, orC₆-C₁₂ n-alkanes.

Alcohols are alkanes where one hydroxy (—OH) group is substituted inplace of one hydrogen of the alkane. Alcohols include, but are notlimited to, C₁-C₂₀ alcohols such as methanol, ethanol, propanol,butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol,undecanol, dodecanol, tridecanol, tetradecanol, pentadecanol,hexadecanol, heptadecanol, octadecanol, nonadecanol, and eicosanol,where the hydroxy group can be located at any carbon atom bonded tothree or fewer other carbon atoms. Alcohols can be straight-chain (e.g.,n-decanol), branched-chain (e.g., t-butanol), or cyclic (e.g.,cyclohexanol); cyclic alcohols can be substituted with n-alkyl and/orbranched alkyl groups, and cyclic alkanes can be substituted with onestraight-chain alcohol or one branched alcohol, plus additional n-alkyland/or branched alkyl groups. In another embodiment, the alkanols areselected from C₄-C₁₆ alkanols or C₆-C₁₂ alkanols. In other embodiments,the alkanols are selected from C₁-C₂₀ n-alkanols (methanol, ethanol,propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol,n-nonanol, n-decanol, n-undecanol, n-dodecanol, n-tridecanol,n-tetradecanol, n-pentadecanol, n-hexadecanol, n-heptadecanol,n-octadecanol, n-nonadecanol, and n-eicosanol), C₄-C₁₆ n-alkanols, orC₆-C₁₂ n-alkanols. In another embodiment, the foregoing groups ofalcohols are 1-ols, that is, the alcohol group is present on the1-carbon or alpha-carbon of the molecule (methanol, ethanol, 1-propanol,1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol,1-decanol, 1-undecanol, 1-dodecanol, 1-tridecanol, 1-tetradecanol,1-pentadecanol, 1-hexadecanol, 1-heptadecanol, 1-octadecanol,1-nonadecanol, and 1-eicosanol). In another embodiment, the alkanols areselected from C₁-C₂₀ n-alkan-1-ols.

In another embodiment, the alkyl-containing molecules are selected fromalkanethiols. Alkanethiols (or alkylthiols) are alkanes where one thiol(sulfhydryl) group (—SH) is substituted in place of one hydrogen of thealkane. Alkanethiols include, but are not limited to, C₁-C₂₀alkanethiols such as methanethiol, ethanethiol, propanethiol,butanethiol, pentanethiol, hexanethiol, heptanethiol, octanethiol,nonanethiol, decanethiol, undecanethiol, dodecanethiol, tridecanethiol,tetradecanethiol, pentadecanethiol, hexadecanethiol, heptadecanethiol,octadecanethiol, nonadecanethiol, and eicosanethiol, where the thiolgroup can be located at any carbon atom bonded to three or fewer othercarbon atoms. Alkanethiols can be straight-chain (e.g., n-decanethiol),branched-chain (e.g., t-butanethiol), or cyclic (e.g.,cyclohexanethiol); cyclic alkanethiols can be substituted with n-alkyland/or branched alkyl groups, and cyclic alkanes can be substituted withone straight-chain alkanethiol or one branched alkanethiol, plusadditional n-alkyl and/or branched alkyl groups. In another embodiment,the alkanethiols are selected from C₄-C₁₆ alkanethiols or C₆-C₁₂alkanethiols. In other embodiments, the alkanethiols are selected fromC₁-C₂₀ n-alkanethiols (methanethiol, ethanethiol, propanethiol,n-butanethiol, n-pentanethiol, n-hexanethiol, n-heptanethiol,n-octanethiol, n-nonanethiol, n-decanethiol, n-undecanethiol,n-dodecanethiol, n-tridecanethiol, n-tetradecanethiol,n-pentadecanethiol, n-hexadecanethiol, n-heptadecanethiol,n-octadecanethiol, n-nonadecanethiol, and n-eicosanethiol), C₄-C₁₆n-alkanethiols, or C₆-C₁₂ n-alkanethiols.

In another embodiment, the foregoing groups of alkanethiols are1-thiols, that is, the thiol group is present on the 1-carbon oralpha-carbon of the molecule (methanethiol, ethanethiol, 1-propanethiol,1-butanethiol, 1-pentanethiol, 1-hexanethiol, 1-heptanethiol,1-octanethiol, 1-nonanethiol, 1-decanethiol, 1-undecanethiol,1-dodecanethiol, 1-tridecanethiol, 1-tetradecanethiol,1-pentadecanethiol, 1-hexadecanethiol, 1-heptadecanethiol,1-octadecanethiol, 1-nonadecanethiol, and 1-eicosanethiol). In anotherembodiment, the alkanethiols are selected from C₁-C₂₀ n-alkane-1-thiols.

In another embodiment, the alkyl-containing molecules are selected fromalkyl halides. Alkyl halides are alkanes where one halide group (—F,—Cl, —Br, —I) is substituted in place of one hydrogen of the alkane.Alkyl halides include, but are not limited to, C₁-C₂₀ alkyl halides suchas halomethane, haloethane, halopropane, halobutane, halopentane,halohexane, haloheptane, halooctane, halononane, halodecane,haloundecane, halododecane, halotridecane, halotetradecane,halopentadecane, halohexadecane, haloheptadecane, halooctadecane,halononadecane, and haloeicosane, where the halide group can be locatedat any carbon atom bonded to three or fewer other carbon atoms. Alkylhalides can be straight-chain (e.g., n-decyl halide), branched-chain(e.g., t-butyl halide), or cyclic (e.g., cyclohexyl halide); cyclicalkyl halides can be substituted with n-alkyl and/or branched alkylgroups, and cyclic alkanes can be substituted with one straight-chainalkane halide or one branched alkane halide, plus additional n-alkyland/or branched alkyl groups. In another embodiment, the alkyl halidesare selected from C₄-C₁₆ alkyl halides or C₆-C₁₂ alkyl halides. In otherembodiments, the alkyl halides are selected from C₁-C₂₀ n-alkyl halides(halomethane, haloethane, halopropane, n-halobutane, n-halopentane,n-halohexane, n-haloheptane, n-halooctane, n-halononane, n-halodecane,n-haloundecane, n-halododecane, n-halotridecane, n-halotetradecane,n-halopentadecane, n-halohexadecane, n-haloheptadecane,n-halooctadecane, n-halononadecane, and n-haloeicosane), C₄-C₁₆ n-alkylhalides, or C₆-C₁₂ n-alkyl halides. In another embodiment, the foregoinggroups of alkyl halides are 1-halo compounds, that is, the halide groupis present on the 1-carbon or alpha-carbon of the molecule (halomethane,haloethane, halopropane, 1-halobutane, 1-halopentane, 1-halohexane,1-haloheptane, 1-halooctane, 1-halononane, 1-halodecane, 1-haloundecane,1-halododecane, 1-halotridecane, 1-halotetradecane, 1-halopentadecane,1-halohexadecane, 1-haloheptadecane, 1-halooctadecane, 1-halononadecane,and 1-haloeicosane). In another embodiment, the alkyl halides areselected from C₁-C₂₀ n-alkane-1-halo compounds. In one embodiment, thehalide group of the foregoing compounds is fluoride. In one embodiment,the halide group of the foregoing compounds is chloride. In oneembodiment, the halide group of the foregoing compounds is bromide. Inone embodiment, the halide group of the foregoing compounds is iodide.

The alkyl-containing molecules can be added to the solution in amountsranging from about 0.1% to about 25%, for example, about 0.1% to about20%, about 0.1% to about 15%, about 0.1% to about 10%, about 0.1% toabout 7.5%, about 0.1% to about 7.5%, about 0.1% to about 7%, about 0.1%to about 6%, about 0.5% to about 15%, about 1% to about 15%, about 1% toabout 10%, about 1% to about 7.5%, about 2% to about 7.5%, about 3% toabout 7%, or about 4% to about 6%, or in amounts of about 0.1%, about0.5%, about 1%, about 2%, about 3%, about 4%, about 5%, about 6%, about7%, about 8%, about 9%, about 12.5%, about 15%, about 17.5%, about 20%,about 22.5%, or about 25%, or in amounts of up to about 0.1%, up toabout 0.2%, up to about 0.5%, up to about 1%, up to about 2%, up toabout 3%, up to about 4%, up to about 5%, up to about 6%, up to about7%, up to about 8%, up to about 9%, up to about 12.5%, up to about 15%,up to about 17.5%, up to about 20%, up to about 22.5%, or up to about25%. The percentages are preferably calculated as volume/volume;weight/weight, weight/volume, or volume/weight can also be used.

Polyfunctional Alkyl-Containing Compounds

Polyfunctional alkyl-containing compounds can also be used in theinvention. In one embodiment, the polyfunctional alkyl-containingcompounds exclude di-halo substituted compounds.

In one embodiment, the alkyl-containing molecules can be substitutedwith at least two functional groups selected from aldehyde (—C(═O)—H),dioxo (i.e., an sp²-hybridized oxygen molecule occupying two valences ona carbon atom to form a ketone, —C(═O)—), hydroxy (—OH), alkoxy(—O—C₁-C₁₂ alkyl), thiol (—SH), thioalkyl (—S—C₁-C₁₂ alkyl), carboxylicacid (—COOH), ester (—C(═O))-O—C₁-C₁₂ alkyl), amine (—NH₂, —NH(C₁-C₁₂alkyl), or —N(C₁-C₁₂ alkyl)₂), amide (—C(═O)—NH₂, —C(═O)—NH(C₁-C₁₂alkyl), or —C(═O)—N(C₁-C₁₂ alkyl)₂), ether (—O—), thioether (sulfide)(—S—), halide (such as fluoride, chloride, bromide, and iodide), nitrile(—CN), epoxide, aromatic, and arylalkyl groups. Aromatic groups include,but are not limited to, C₆-C₁₀ aryl groups, such as phenyl, benzyl, andnaphthyl groups. Arylalkyl (or aralkyl) groups include, but are notlimited to, C₀-C₆alkyl-C₆-C₁₀ aryl-C₀-C₆ alkyl groups. In oneembodiment, the polyfunctional alkyl-containing compounds excludedi-halo substituted compounds.

In one embodiment, the polyfunctional alkyl-containing compounds areselected from diols, polyols, dithiols, polythiols, and mixedalcohol-thiol compounds. These polyfunctional alkyl-containing compoundsinclude, but are not limited to, C₁-C₂₀ alkyl molecules substituted withat least two functional groups, where each of the at least twofunctional groups are independently chosen from —SH and —OH. The C₁-C₂₀alkyl to be substituted with the at least two functional groups can be astraight-chain, branched-chain, or cyclic alkyl, or a combination ofstraight-chain, branched-chain, and cyclic alkyl moieties. The at leasttwo functional groups chosen from —SH and —OH can be substituted ontoany carbon atom in the alkyl moiety. In one embodiment, the at least twofunctional groups chosen from —SH and —OH are substituted on differentcarbon atoms; in this embodiment, there must be at least as many carbonatoms as there are total —SH and —OH groups in the molecule. In anotherembodiment, the polyfunctional alkyl-containing compounds are selectedfrom compounds of the formula C_((i+j)) alkyl to C₂₀ alkyl, where i iszero or a positive integer and designates the number of hydroxy groups,j is zero or a positive integer and designates the number of thiolgroups, and (i+j) is an integer between 2 and 20, between 2 and 10,between 2 and 6, or between 2 and 4.

In one embodiment, the polyfunctional alkyl-containing compounds aredi-substituted compounds with each substitution on a different carbonatom, such as diols (corresponding to the immediately precedingembodiment with i=2 and j=0), di-thiols (corresponding to theimmediately preceding embodiment with i=0 and j=2), or analkyl-containing compound with one —OH group and one —SH group(corresponding to the immediately preceding embodiment with i=1 andj=1). In one embodiment of these di-substituted compounds, thealkyl-containing compound is an n-alkyl compound, that is, C₂-C₂₀n-alkyl substituted with two —OH groups, two —SH groups, or one —OHgroup and one —SH group. In one embodiment of the di-substituted n-alkylcompounds, the substituents are in the alpha and omega positions, i.e.,on the first and last, or terminal, atoms of the n-alkyl chain; that is,C₂-C₂₀ n-alkyl with alpha, omega substituted —OH groups; or C₂-C₂₀n-alkyl with alpha, omega substituted —SH groups; or C₂-C₂₀ n-alkyl withan alpha —OH group and an omega —SH group (as an n-alkyl group issymmetric, an n-alkyl group with an alpha —OH group and an omega —SHgroup is equivalent to an n-alkyl with an omega —OH group and an alpha—SH group). In another embodiment, the C₂-C₂₀ n-alkyl with alpha, omegasubstituted —SH groups are selected from 1,3-propanedithiol,1,4-butanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol,1,7-heptanedithiol, 1,8-octanedithiol, and 1,9-nonanedithiol.

The polyfunctional alkyl-containing molecules can be added to thesolution in amounts ranging from about 0.1% to about 25%, for example,about 0.1% to about 20%, about 0.1% to about 15%, about 0.1% to about10%, about 0.1% to about 7.5%, about 0.1% to about 7.5%, about 0.1% toabout 7%, about 0.1% to about 6%, about 0.5% to about 15%, about 1% toabout 15%, about 1% to about 10%, about 1% to about 7.5%, about 2% toabout 7.5%, about 3% to about 7%, or about 4% to about 6%, or in amountsof about 0.1%, about 0.5%, about 1%, about 2%, about 3%, about 4%, about5%, about 6%, about 7%, about 8%, about 9%, about 12.5%, about 15%,about 17.5%, about 20%, about 22.5%, or about 25%, or in amounts of upto about 0.1%, up to about 0.2%, up to about 0.5%, up to about 1%, up toabout 2%, up to about 3%, up to about 4%, up to about 5%, up to about6%, up to about 7%, up to about 8%, up to about 9%, up to about 12.5%,up to about 15%, up to about 17.5%, up to about 20%, up to about 22.5%,or up to about 25%. The percentages are preferably calculated asvolume/volume; weight/weight, weight/volume, or volume/weight can alsobe used.

Organic Semiconductor Films

The organic semiconductor film is typically fabricated with an electrondonor material and an electron acceptor material, which allows anelectron-hole pair (an exciton) generated by a photon to separate andgenerate current. The junction between the donor and acceptor can becreated by forming a layer of one material (e.g., the donor) on top ofthe other material (e.g., the acceptor), which forms a planarheterojunction between the bilayers. Interpenetrating networks of donorand acceptor materials can be used, which increase the surface area ofthe heterojunction, while providing paths for electrons or holes totravel without recombining. Interpenetrating networks can be formed bydiffuse interfaces at a bilayer heterojunction, where portions of donormaterial and acceptor material extend into the other material near thebilayer junction. Bulk heterojunctions can be formed when the donor andacceptor materials are mixed together to form a multi-component activelayer. In one embodiment, the organic semiconductor films of theinvention are formed as bulk heterojunctions.

In one embodiment, only about 10% or less, about 5% or less, about 2% orless, about 1% or less, about 0.5% or less, about 0.1% or less, about0.05% or less, about 0.01% or less, about 0.005% or less, or about0.001% or less of the alkyl-containing molecule additive remains in theorganic semiconductor film after fabrication, relative to the originalamount of the additive in the solution used to form the semiconductorfilm. In another embodiment, the alkyl-containing molecule makes up onlyabout 10% or less, about 5% or less, about 2% or less, about 1% or less,about 0.5% or less, about 0.1% or less, about 0.05% or less, about 0.01%or less, about 0.005% or less, or about 0.001% or less by weight of thefinal organic semiconductor film.

Donor Materials

1) Conjugated Organic Polymers

Conjugated organic polymers typically function as the electron-donatingmaterial in the organic semiconductor films, such as in bulkheterojunction films. Examples of suitable conjugated organic polymerswhich can be used are provided in U.S. Patent Application PublicationNo. 2005/0279399, which is hereby incorporated by reference in itsentirety, particularly with regards to its discussion of conjugatedorganic polymers. Examples of such conjugated polymers include one ormore of polyacetylene, polyphenylenes, poly(3-alkylthiophenes) wherealkyl is from 6 to 16 carbons (P3AT's) such as poly-(3-hexylthiophene)(P3HT),poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT), polyphenylacetylene, polydiphenylacetylene, polyanilines,poly(p-phenylene vinylene) (PPV) and alkoxy derivatives thereof such aspoly(2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) andpoly(2,5-dimethoxy-p-phenylene vinylene) (PDMPV), polythiophenes,poly(thienylenevinylenes) such as poly(2,5-thienylenevinylene),polyfluorenes, polyporphyrins, porphyrinic macrocycles,thiol-derivatized polyporphyrins, polymetallocenes such aspolyferrocenes, polyphthalocyanines, polyvinylenes, polyphenylvinylenes,polysilanes, polyisothianaphthalenes, polythienylvinylenes, derivativesof any of these materials and blends or combinations thereof in anyproportion. Exemplary derivatives of these materials include derivativeshaving pendant groups, e.g., a cyclic ether, such as epoxy, oxetane,furan, or cyclohexene oxide. Derivatives of these materials mayalternatively or additionally include other substituents. For example,thiophene components of electron donor may include a phenyl or alkylgroup, such as at the 3 position of each thiophene moiety. Examples ofsuch thiophenes are thiophenes of the form

where R₃ is C₁-C₈ alkyl or C₁-C₆ alkyl-C(═O)—O—C₁-C₆ alkyl, such as:

(poly[3-(ethyl-4-butanoate)thiophene-2,5-diyl]). As another example,alkyl, alkoxy, cyano, amino, and/or hydroxy substituent groups may bepresent in any of the polyphenylacetylene, polydiphenylacetylene,polythiophene, and poly(p-phenylene vinylene) conjugated polymers.Acceptor Materials1) Fullerenes

Fullerene compounds typically function as the electron-acceptingmaterial in the heterojunction. Exemplary fullerene derivatives for usein the invention include[6,6]-phenyl C61-butyric acid methyl ester(PCBM) and C71-PCBM. Additional commercially available fullerenederivatives which can be used in the invention are shown in the tablebelow.

Name and Common Abbreviation Structure Phenyl-C₆₁- Butyric-Acid- MethylEster, [60]PCBM

Phenyl-C₇₁- Butyric-Acid- Methyl Ester, [70]PCBM

Phenyl-C₈₅- Butyric-Acid- Methyl Ester, [84]PCBM

Phenyl-C₆₁- Butyric-Acid- Butyl Ester, PCBB, [60]PCB-C4

Phenyl-C₆₁- Butyric-Acid- Octyl Ester, PCBO, [60]PCB-C₈

Thienyl-C₆₁- Butyric-Acid- Methyl Ester, [60]ThCBM,

In general, fullerenes which can be used in the invention are of theform Ar—C(=fullerene)-R₁—C(═O)—R₂, diagrammed below, where Ar is phenylor thienyl, which can be optionally substituted, R₁ is C₁-C₁₂ alkyl(preferably C₄ alkyl) and R₂ is —O—C₁-C₁₂ alkyl or —O—C₁-C₁₂ alkyl-SH.The optional substituents on Ar include, but are not limited to, C₁-C₁₂alkyl, F, Cl, Br, I, —O—C₁-C₁₂ alkyl, and other substituents asindicated above for the low molecular weight alkyl compounds whenchemically appropriate. “Fullerene” is selected from a C60, C70, or C84fullerene moiety.

2) Carbon Nanotubes

Carbon nanotubes can also be used as electron acceptors in devicesfabricated with the organic semiconducting films disclosed herein. See,for example, Harris, P. J. F., “Carbon Nanotubes and RelatedStructures,” Cambridge University Press, 2002; Dresselhaus, M. S. etal., “Carbon Nanotubes,” Springer-Verlag, 2000; U.S. Pat. No. 4,663,230;Guldi, D. et al., Chemical Society Reviews (2006), 35(5), 471-487;Kymakis, E. et al., Optical Science and Engineering (2005), 99(OrganicPhotovoltaics), 351-365; and Guldi, D. et al., Accounts of ChemicalResearch (2005), 38(11), 871-878. Single-walled carbon nanotubes andmulti-walled carbon nanotubes are available commercially from a varietyof vendors, such as Carbon Solutions, Inc. of Riverside, Calif. andHelix Material Solutions of Richardson, Tex.

3) Other Acceptor Materials

Other materials, such as electron-deficient polymers andelectron-deficient molecules, can be used as electron acceptors.Examples of such materials include, but are not limited to,2,7-poly-(9-fluorenone) (2,7-PFO), pi-conjugated organoboron polymers(U.S. Patent Application Publication No. 2007/0215864),poly(para-phenylene vinylene) modified with cyano groups (Granstrom etal. Nature 395, 257-260, 1998); pi-conjugated oxadiazole-containingpolymers (Li et al. J. Chem. Soc. Chem. Commun. 2211-2212, 1995),pi-conjugated quinoxaline-containing polymers, and pi-conjugatedpolymers incorporating regioregular dioctylbithiophene andbis(phenylquinoline) units in the backbone of the polymer (Babel, A.,Jenekhe, S. A. Adv. Mater., 14, 371-374, 2002).

Solvents

Any suitable solvent can be employed for the solution used to form theconjugated polymer film. A suitable solvent is a solvent in which thecomponents of the film, such as the conjugated polymer, fullerenederivative, and alkyl containing molecule can be dissolved, dispersed orsuspended, and from which a conjugated polymer film with the desiredproperties of photoconductivity can be formed. Solvents including, butnot limited to, halogenated alkanes and aromatic and halogenatedaromatic compounds, such as chloroalkanes and chlorine-bearing aromaticcompounds, can be used in the invention. Examples of suitable solventsinclude, but are not limited to, chlorobenzene, dichlorobenzene,trichlorobenzene, benzene, toluene, chloroform, dichloromethane,dichloroethane, xylenes such as o-xylene, m-xylene, p-xylene, andmixtures thereof, α,α,α-trichlorotoluene, methyl naphthalene,chloronaphthalene, and mixtures thereof.

Formation of Organic Semiconductor Films

The solution used to form the organic semiconductor films, such asconjugated polymer films, can be deposited on a substrate, such as atransparent support or an electrode, by a variety of methods. Suchmethods include spin casting, “doctor blading,” drop-casting, sequentialspin-casting, formation of Langmuir-Blodgett films, electrostaticadsorption techniques, and dipping the substrate into the solution.Subsequent processing steps can include evaporation of the solvent toform the film, optionally under reduced pressure and/or elevatedtemperature; and thermal annealing of the deposited film.

Devices Formed Using Organic Semiconductor Films

A variety of devices can be formed using organic semiconductor films,such as conjugated polymer films, produced with the methods of theinvention. Such devices include solar cells, photovoltaic cells,photodetectors, photodiodes, phototransistors, and thin filmtransistors.

In one embodiment, the devices comprise a first electrode, a secondelectrode, and an organic semiconductor film between the first andsecond electrode. The first and second electrodes should have differingwork functions; the electrode with the higher work function isdesignated the high work function electrode while the electrode with thelower work function is designated the low work function electrode. Thedevice can comprise an electrode made out of a material such astransparent indium-tin oxide (ITO) deposited on glass (where ITO servesas the high work function electrode or hole-injecting electrode), theorganic semiconductor film, and an electrode made out of a material suchas aluminum (where Al serves as the low work function, orelectron-injecting, electrode).

In one embodiment, the high work function electrode has a work functionat or above about 4.5 electron volts. The high work function electrodeis typically a transparent conductive metal-metal oxide or sulfidematerial such as indium-tin oxide (ITO) with resistivity of 20ohm/square or less and transmission of 89% or greater at 550 nm. Othermaterials are available such as thin, transparent layers of gold orsilver. This electrode is commonly deposited on a solid support bythermal vapor deposition, electron beam evaporation, RF or Magnetronsputtering, chemical vapor deposition or the like. (These same processescan be used to deposit the low work-function electrode as well.) Theprincipal requirement of the high work function electrode is thecombination of a suitable work function, low resistivity and hightransparency. In another embodiment, the low work function electrode hasa work function at or below about 4.3 eV; examples of such materialsinclude aluminum, indium, calcium, barium and magnesium. Either of theelectrodes can be fabricated by deposition onto a support, for example,indium-tin oxide deposited on a glass or plastic substrate, or theelectrodes can be fabricated without a support.

The invention will be further understood by the following nonlimitingexamples.

EXAMPLES Example 1 Fabrication and Evaluation of Cells

PCBM was synthesized as described in the literature (J. C. Hummelen, B.W. Knight, F. LePeq, F. Wudl, J. Org. Chem. 1995, 60, 532), P3HT waspurchased from Rieke metals, Baytron P was purchased from H. C. Starck,and all other chemicals were purchased from Sigma/Aldrich and were usedas received. Glass slides coated with indium tin oxide were rinsed withsoap and water and sonicated once in soapy water, three times indeionized water, and once in each of isopropyl alcohol and acetone forten minutes prior to being dusted with a nitrogen gun and driedovernight. Prior to spin coating samples were cleaned under UV/Ozone for30 minutes and dusted again with nitrogen. A layer of Baytron PPoly(ethylene dioxythiophene):Poly(styrene sulfonate) was spin coated atroom temperature under atmosphere at 5000 RPM for one minute and thendried at 120° C. for 15 minutes. Devices were then transferred to aglove box which is maintained at less than 5 ppm of O₂. Active layersolutions were prepared by dissolving 1% P3HT and 0.8% PCBM by weight intoluene and allowing the solutions to stir at 70° C. overnight. Alkylthiol solutions were prepared by adding the appropriate volume of alkylthiol to the solvent prior to the dissolution of the P3HT and PCBM. Theactive layers were applied by spin coating at 700 RPM for one minute.The devices were then moved into a second glove box maintaining anoxygen concentration of less than 1 ppm where the aluminum cathode wasdeposited at a pressure of less that 1×10⁻⁶ ton at an initial rate of nomore than 0.15 nm/sec to a total thickness of at least 120 nm. Annealingwas conducted at 150° C. for 15 minutes by placing the devices on ahotplate. The same general technique was used to fabricate cells in therest of the Examples, with variations in the components as indicated ineach Example.

Efficiency testing was conducted as described previously (U. Zhokhavets,T. Erb, H. Hoppe, G. Gobsch, N. S. Sariciftci, Thin Solid Films 2006,496, 679). TEM images obtained at 80 kV on a Philips CM 10 using coppergrids ordered from Ted Pella Inc. with samples prepared as describedabove. Optical transmission studies were conducted by spin coating thedevice active layers on glass and scanning in a Shimadzu UV-2401PCUV-Visible spectrophotometer. Atomic force microscopy was conducted on aVeeco multimode AFM in tapping mode where thicknesses were determined bythe scratch method.

The samples for steady-state and transient photoconductivitymeasurements were spin cast on alumina substrates and the electrodeswere deposited in the surface-contact, Auston switch configuration (D.H. Auston, IEEE Journal of Quantum Electronics 1983, 19, 639).Steady-state photoconductivity was measured by a standard modulationtechnique (C. H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N. S.Sariciftci, A. J. Heeger, F. Wudl, Phys. Rev. B 1993, 48(20) 15425),where the excitation was provided by a Xenon lamp, the modulationfrequency was 166 Hz and the applied electric field was E=5 KV/cm. Allthe measurements were performed while the samples were kept in vacuum(P<10⁻⁴ Torr).

For convenience, samples are designated by their composition and thermalhistory. Samples are designated bulk heterojunction, or BHJ, whichindicates a 10:8 ratio of P3HT and PCBM spun cast from toluene at 700RPM. The BHJ is followed by a concentration number and alkyl thiol chainlength; for example, BHJ-5% C8 is a 10:8 P3HT:PCBM film spun cast from atoluene solution containing 5% by volume octanethiol. Unless otherwisedesignated, PCBM indicates C60-PCBM. After the composition of thesolvent mixture, there will either be an A or an NA to designate if thesample has been annealed or not, respectively. For this work, the devicefabrication method described by W. Ma et al., Adv. Func. Mater. 2005,15, 1617, was used, except that toluene was used as the solvent. Deviceshad an active layer thickness of about 100 nm, as measured by atomicforce microscopy (AFM). Under a calibrated AM 1.5 solar illuminationsource, the control devices were, on average, 2% efficient prior toannealing (BHJ-NA) and 3.3% efficient after annealing at 150° C. for 15minutes (BHJ-A).

FIG. 1 displays the differences in the transmittance of BHJ-0.75% C8-NAand BHJ-0.75% C8-A, as compared to BHJ-NA and BHJ-A. Incorporation ofn-octanethiol to the P3HT/PCBM/toluene solution (between 0.1% and 10% byvolume) leads to a decrease of the transmittance, similar to thatobserved after thermal treatment (BHJ-A) (W. Ma et al., Adv. Func.Mater. 2005, 15, 1617; D. Chirvase et al., Nanotechnology 2004, 15,1317). The enhancement of the transmittance of BHJ-0.75% C8-NA relativeto BHJ-NA may arise from increased optical absorption due to morepronounced interchain interactions, leading to absorption of incidentlight perpendicular to the substrate or enhanced light scattering in thefilm; the latter process might also contribute also to increasedabsorption as well due to multiple scattering in the active layer (T. Q.Nguyen et al., J. Phys. Chem. B. 2000, 104, 237). A similar decrease intransmission can be observed by mixing octanol or undecane in thetoluene solution prior to spin casting. Due to the fact that a similareffect is observed for both nonpolar and polar additives, it seemsunlikely that the changes in transmittance are due to solvatochromiceffects resulting from residual solvent molecules.

Steady-state photoconductivity measurements were performed as a functionof annealing (FIG. 2 a), alkyl chain length (FIG. 2 b), and alkyl thiolconcentration (FIG. 2 c). The photoconductive response is presented interms of sample responsivity (R), i.e. photocurrent/incident light power(mA/W), as measured using an external field of F=5 KV/cm. FIG. 2( a)shows that while annealing increases the responsivity of sample BHJ-NAby a factor of 6, inclusion of 5% n-octanethiol in solution combinedwith annealing increases the responsivity of sample BHJ-NA by a factorof 60. FIG. 2( b) shows that utilizing n-octanethiol results in higherresponsivity than n-dodecanethiol or n-hexanethiol; FIG. 2( c) showsthat among the concentrations of n-octanethiol tested, the sampleobtained from a 5% by volume solution led to the highest responsivity.

The steady-state photoconductivity is given in Equation 3, where e isthe charge of an electron, n is the number of either electrons or holes,μ is the carrier mobility, and τ is the carrier lifetime (C. H. Lee etal., Synt. Met., 1995, 70, 1353; D. Moseset al., Synt. Met., 1997, 84,539; C. Y. Yang et al., Synt. Met., 2005, 155, 639).σ=e[(n _(h)μ_(h)τ_(h))+(n _(e)μ_(e)τ_(e))]  (Equation 3)

Photoconductivity of pristine P3HT has previously been shown to increaseby more than two orders of magnitude with the addition of the fullerenedue to the ultrafast transfer of photoexcited electrons from the P3HT tothe fullerene, which increases the number of separated carriers (n) andcarrier life time. Upon annealing, a further increase inphotoconductivity by a factor of 6 was observed.

From the maximum responsivity measured in BHJ-5% C8-A at 590 nm, andtaking into account the device cross sectional area of 6×10⁻⁷ cm², aphotocurrent density per Watt of incident radiation of J=4×10⁵ A/cm² wasestimated. This magnitude of J corresponds to a photoconductivity perWatt of incident radiation of σ=200 S/cm at E=5 KV/cm. Assuming that thenumber of carriers scales with optical absorption and given that thetransmission spectra of the BHJ-A samples are similar to thethiol-modified samples, it follows from Equation 3 that processing withalky thiols modifies the product of carrier mobility and carrierlifetime.

To disentangle the effects of the alkyl thiols on the charge carriermobility and carrier lifetime, fast (t≧100 ps) transientphotoconductivity measurements were performed from which the lifetimewas directly determined. The measured carrier lifetime increases from 36ns in BHJ-NA to 65 ns in BHJ-A. This corresponds to a factor of 1.8improvement in the carrier lifetime due to annealing, while adding alkylthiol to the solution increased the lifetime by only a factor of 1.3.Both the magnitude of the transient photocurrent waveforms and thephotocurrent decay rate are consistent with the magnitude of thesteady-state photocurrents. Thus, annealing and processing using alkylthiol result in similar increases in the carrier lifetime; however,since the photocurrent measured in BHJ-C8-NA was significantly higherthan that in the BHJ-A sample, it can be concluded that the carriermobility is increased by processing with alkyl thiol.

Measurements of the photovoltaic device efficiency were conducted tocharacterize the effects of the thiol solution on solar cell deviceperformance. FIG. 3 shows how the power conversion efficiency (η_(e)) isaffected by the inclusion of alkyl-thiol molecules, as compared toBHJ-NA and BHJ-A samples. Maximum increase in efficiency occurred at analkyl thiol concentration of 0.75 vol %. At this concentration, theefficiency increased by 50% over the control device prior to thermalannealing and the efficiency then increased by 20% after thermalannealing. Note that the improvements relative to the control samplesshown in FIG. 3 are smaller than the ones observed in thephotoconductivity measurements in FIG. 2.

It is interesting to note that photoconductivity improvements do notnecessarily translate directly to solar cell efficiency improvements.One possible reason to account for this difference is that spinning froma solution containing thiol yields films which have greatly improvedcharge transport properties in the direction parallel to the substrate,but not in the direction perpendicular to the substrate. Because thephotoconductivity studies were conducted with surface electrodes,whereas solar cell devices are operated through the thickness of thefilm, anisotropy of charge transport properties could account for thedifference in photoconductivity enhancement and device efficiencyenhancement. Another possible explanation is that because this solarcell device structure has been optimized such that the thickness iscomparable to the carrier diffusion length, device efficiency may not begreatly limited by carrier mobility.

To test the extent to which carrier diffusion length influencesefficiency, devices were prepared with twice the concentration of P3HTand PCBM and 1 vol % n-octanethiol to create a thicker device activelayer (W. Ma et al., Adv. Func. Mater. 2005, 15, 1617; G. Li et al., J.Appl. Phys., 2005, 98, 043704). The devices were approximately 200 nmthick and showed increased short circuit currents due to increasedabsorption. Device efficiencies were 3.6% before annealing and 4.0%after annealing. These values should be compared to a 100 nm thickcontrol which was 2.0% efficient before annealing and 3.3% efficientafter annealing. Due to the decrease in the built-in field, however, theV_(oc) dropped to 0.58 V from 0.61 V and the fill factor dropped to 59%from 65%.

As indicated above, peak device efficiency occurs slightly under 1% byvolume of alkyl thiol in solution for 100 nm thick devices. The reasonfor the disparity in optimum alkyl thiol concentrations between theefficiency measurements and the photoconductivity measurements isbelieved to be due to excess alkyl thiol being squeezed to the surfacein the higher concentration samples. Examination of the films by AFMshowed droplets on the surface of the films from solutions with alkylthiol concentrations greater than one percent by volume. The surfacecontact geometry used in the photoconductivity setup would be lesssensitive to islands of poor conductivity between the depositedelectrode and the active layer.

One possible explanation for the effects observed is that alkyl chainsact as a compatibilizer by lubricating the P3HT chains during the highlydynamic and kinetically limited spin coating process. This processingtechnique can be used as a tool elucidate the factors that affectinterchain order in the P3HT phase and the blend morphology of P3HT andPCBM mixtures.

The following table summarizes current-voltage characteristic of solarcells made of regio-regular polythiophene (P3HT)/PCBM cast fromchlorobenzene, with and without a solvent additive.

Thickness V_(oc) I_(sc) FF PCE (nm) (V) (mA/cm²) (%) (%) Solvent 120 NA0.6  −5.6 0.48 2.1 CB 120 NA 0.53 −8.6 0.61 3.5 CB/1% C8SH 120 A 0.65−8.1 0.61 4.0 CB 120 A 0.64 −8.5 0.64 4.4 CB/1% C8SH 220 NA 0.65 −4 0.381.3 CB 220 NA 0.52 −10.4 0.51 3.4 CB/2.5% C8SH 220 A 0.62 −9.4 0.53 3.9CB 220 A 0.63 −10.8 0.57 4.8 CB/2.5% C8SH

Example 2 Thin Film Transistors

FIG. 4 shows the behavior of transistors fabricated from semiconductorfilms (BHJ-NA, dashed lines, and BHJ-5% C8-NA, solid lines). TFT deviceswere spun at 2000 rpm on heavily doped n-type Si coated with a 200 nmsilica dielectric layer treated with octyltrichlorosilane. The sourceand drain electrodes comprised of 50 nm thick Au deposited on a 5 nmthick Ti adhesion layer in a bottom contact geometry; the TFT channellengths were 5 or 10 μm and the channel width was 1 mm. Films fabricatedwith addition of 5% octanethiol show dramatic increases in current ateach voltage tested.

Example 3 X-Ray Diffraction Results

FIG. 5 and FIG. 30 show the results of X-ray diffraction studies onfilms processed with and without additive and with or without annealing.Films for X-ray diffraction were spun on glass substrates. Bothannealing and use of additive induce significant changes in the X-raydiffraction pattern as compared to the non-annealed film cast withoutadditive, indicative of underlying changes in film structure. FIG. 5depicts X-ray diffraction results highlighting the <100> diffractionpeak for P3HT in P3HT/PCBM films cast from toluene BHJ-NA (x's, X),BHJ-A (asterisks or *,

), BHJ-0.75% C8-NA (filled diamonds, ♦), and BHJ-0.75% C8-A (opendiamonds, ⋄). FIG. 30 depicts X-ray diffraction results highlighting the<100> diffraction peak for P3HT in P3HT/C-60 PCBM films cast fromchlorobenzene (squares) or chlorobenzene containing 2.5% octanethiol(circles) either before annealing (closed shapes) or after annealing(open shapes). The films are cast using toluene (FIG. 5) andchlorobenzene (FIG. 30), indicating that the solvent from which thefilms are cast also affects the structure of the films.

Example 4 Photocurrent Transients

FIG. 6 shows the transient photocurrent induced in films processed withand without additive. The additive-processed film shows a highertransient photocurrent following the decay from the initial peak. Thephotoconductive response (R), in terms of photocurrent per incidentlight power (A/W), was measured using an external field of F=5 KV/cm.Measurements of the transient photoconductivity employed an Austonswitch sample configuration (D. H. Auston, IEEE Journal of QuantumElectronics 19, 639 (1983)).

Example 5 Power Conversion Efficiency

FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIG. 13, FIG. 14,FIG. 15, FIG. 16, FIG. 17, FIG. 18, FIG. 19, FIG. 20, FIG. 21, FIG. 22,FIG. 23, FIG. 24, FIG. 25, FIG. 26, and FIG. 27 display the effect ofusing various additives according to the invention on the solar cellpower conversion efficiency. As displayed in the Figures, powerconversion efficiency can be affected by the chemical nature of theadditive, the percentage of additive in the casting solution, thesolvent used for casting, annealing the film versus not annealing thefilm, the type of donor or acceptor semiconductor used, and even thespin rate used during spin-casting.

Example 6 Solar Cell Current Density

FIG. 28 shows the increase in current density at various voltages forP3HT/PCBM films cast from chlorobenzene, with and without 2.5%octanethiol. Use of additive shows an increase in current densitysimilar to that seen with annealing.

Example 7 External Quantum Efficiency

FIG. 29 shows the effect of processing with phenyl hexane on theexternal quantum efficiency (EQE) of P3HT:C60-PCBM devices. The thicksolid line correspond to films cast without additive; the thick dottedline shows EQE after annealing. The thin solid line correspond to filmscast with 1% phenyl hexane, while the thin dotted line shows EQE of theadditive-processed film after annealing.

Example 8 Carrier Mobility

FIG. 31 shows the effect of additives on hole and electron mobility whendifferent contact metals are used for the devices. Films contacting bothgold and aluminum show dramatically increased hole mobility whenprocessed with additives according to the invention. Additives alsoincrease electron mobility as show with aluminum contact metal.

Example 9 Effects on Absorption

FIG. 32 and FIG. 33 show the effect of additives on maximum peakabsorbance values and absorption values at 600 nm, respectively. Filmsfor optical transmission were spun on glass substrates. The variousadditives generally increase absorbance of the non-annealed filmsrelative to control (non-additive processed film); some additives alsoincrease absorbance of the annealed films relative to control. Theincrease in absorption can lead to increased power output per unit inputof solar light.

Example 10 Effects on Steady-State Photoconductivity

FIG. 34 shows steady state photoconductivity of P3HT:C60-PCBM (circles)and PCPDTBT:C70-PCBM (squares) processed from chlorobenzene (solidshapes) and with thiol (open shapes). The additive-processed films showdramatic increases in photoresponsivity at wavelengths below 600 nm, ascompared to films processed without additive. The photoconductiveresponse (R), in terms of photocurrent per incident light power (A/W),was measured using an external field of F=5 KV/cm. Measurements of thesteady-state photoconductivity employed a standard modulation technique(C. H. Lee et al., Phys. Rev. B 48(20) 15425 (1993)).

Example 11 Fabrication and Evaluation of Cells for Alkyl DithiolExperiments

Photovoltaic cells were fabricated by spin-casting the active bulkheterojunction layer onto a 60 nm layer of H. C. Stark Baytron PPEDOT:PSS on Corning 1737 AMLCD glass patterned with 140 nm of indiumtin oxide (ITO) by Thin Film Devices Inc. A 100 nm thick aluminumcathode was deposited (area 17 mm²) by thermal evaporation with noheating of the sample either before or after deposition. Unlessotherwise stated, the bulk heterojunction layer was spin cast at 1200RPM from a solution of 2.4 mg/mL octanedithiol in CB containing 10 mg/mLPCPDTBT and 20 mg/mL C₇₁-PCBM. PCPDTBT was obtained from Konarka Inc.and the C₇₁-PCBM was purchased from Nano-C Inc. Atomic force microscopy(AFM) showed that the active layers were approximately 110 nm thickregardless of alkanedithiol concentration in the solution.

Device efficiencies were measured with a 150 Watt Newport-Oriel AM 1.5 Glight source operating at 80 mW/cm² and independently cross-checkedusing a 300 Watt AM 1.5 G source operating at 100 mW/cm² forverification. Solar simulator illumination intensity was measured usinga standard silicon photovoltaic with a protective KG5 filter calibratedby the National Renewable Energy Laboratory (NREL). Many of the mostefficient devices were fabricated independently by different individualsin two separate laboratories and cross-checked under the two differentillumination sources. Incident photon conversion efficiency (IPCE)spectra measurements were made with a 250 W Xe source, a McPhersonEU-700-56 monochromator, optical chopper and lock-in amplifier, and aNIST traceable silicon photodiode for monochromatic power densitycalibration.

Photoconductive devices were fabricated by spin casting on aluminasubstrates as described previously. For steady-state photoconductivitymeasurements, the standard modulation technique was used; for transientphotoconductivity, an Auston switch configuration was used. TFT deviceswere fabricated and tested as described previously with in a bottomcontact geometry with gold electrodes. AFM images were taken on a Veecomultimode AFM with nanoscope Ma controller. UV-Vis absorptionspectroscopy was measured on a Shimadzu 2401 diode array spectrometer.XPS spectra were recorded on a Kratos Axis Ultra XPS system with a basepressure of 1×10⁻¹⁰ mbar using a monochromated Al Kα X-ray source. XPSsurvey scans were taken at 160 pass energy and high resolution scanswere taken at 10 pass energy. Data analysis was done with the CASA XPSsoftware package.

Example 12 Results of Alkyldithiol Experiments

FIG. 35 shows the shift in the film absorption caused by addingdifferent alkanedithiols to the PCPDTBT:C₇₁-PCBM solution inchlorobenzene (CB) prior to spin casting. The largest change is observedwith the addition of 2.4 mg/mL of 1,8-octanedithiol into the CB; thefilm absorption peak red-shifts 41 nm to 800 nm. Such a shift to lowerenergies and the emergence of structure on the absorption peakassociated with the π-π* transition when films are processed withalkanedithiols indicates that the PCPDTBT chains interact more stronglyand that there is improved local structural order compared with filmsprocessed from pure CB. Analysis by Fourier-Transform Infrared (FTIR)and Raman spectroscopies yielded no resolvable thiol signals afterdrying in a low vacuum (˜10⁻³ torr) for 10 minutes at room temperature(when FTIR was measured on wet films prior to exposure to a vacuum, asmall thiol peak can be observed). X-ray photoelectron spectroscopy(XPS) averaged over several samples with multiple scans per sampleindicates no significant content of dithiol, certainly less than 0.1dithiol per PCPDTBT repeat unit after thorough drying in vacuum.

The current voltage characteristics obtained under 80 mW/cm² for devicesprocessed from CB and from CB with 2.4 mg/mL by volume alkanedithiolswith different chain lengths are shown in FIG. 36. From the I-V curvesin FIG. 36, it is apparent that processing with 1,8-octanedithiolincreases both I_(sc) and FF.

Device optimization involved over 1000 devices made from over 250independently prepared PCPDTBT:C₇₁-PCBM films; optimum photovoltaicefficiencies between 5.2% and 5.8% were obtained. The most efficientdevices comprised a polymer/fullerene ratio between 1:2 and 1:3, a spinspeed between 1200 and 1600 RPM, a polymer concentration of between 0.8and 1% by weight, and a 1,8-octanedithiol concentration of between 1.75and 2.5 mg/mL. The most repeatable series of high efficiency devices hadan average power conversion efficiency of 5.5% under 100 mW/cm², withshort circuit current I_(sc)=16.2 mA/cm², FF=0.55, and open circuitvoltage V_(∂)=0.62 V. More than 40 devices gave efficiencies over 5.2%.Nevertheless, as implied by the measured FF=0.55, there are significantimprovements to be made that could lead to more efficient solar energyconversion.

As shown in FIG. 37, the IPCE spectra of photovoltaic cells made withfilms cast from CB compare very well with those previously reported forPCPDTBT:C₇₁-PCBM films formed by “doctor blade” deposition fromortho-dichlorobenzene. Devices processed with 1,8-octanedithiolincreased the IPCE by a factor of approximately 1.6 between 400 nm and800 nm over devices processed without the additive. The integrated IPCEand the I_(sc) measured on the same device, also shown in FIG. 37, agreeto within approximately 5%. Note that since the IPCE for P3HT:C₆₁-PCBMis limited to a narrower fraction of the solar spectrum, higherefficiencies are anticipated for the PCPDTBT:C₇₁-PCBM devices.

To determine the effect of the alkanedithiol processing on carriertransport, the steady-state and transient photoconductivity was measuredin films fabricated with and without processing with the alkanedithioladditive. The steady-state responsivity at E=20 KV/cm (E is the appliedelectric field) and the magnitude of the current response in thetransient photoconductivity from films processed using 1,8-octanedithiolare each larger than the values obtained from films processed withoutdithiols by approximately a factor of two. The measured current in thetransient data indicates an increase in the number of extracted mobilecarriers, and the waveform indicates an additional increase in thecarrier lifetime. As in the absorption, a red-shift is observed in thepeak photoconductivity from 765 nm to 825 nm with the use ofalkanedithiols.

Changes in the surface topography of films cast from pure chlorobenzene(CB) and from CB with 2.4 mg/mL of the alkanedithiols were examined byAFM. Comparison of the data (not shown) obtained from films processedwith 1,4-butanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol or1,9-nonanedithiol indicate that approximately six methylene units arerequired for the alkanedithiol to have an appreciable effect on themorphology; however, as seen in FIG. 36, devices made from films castwith 1,4-butanedithiol showed improvement in current-voltagecharacteristics. Corresponding changes in the internal nano-structurewere also seen by Transmission Electron Microscopy (TEM). The strongdependence of the absorption, the morphology and the device performanceon the alkyl chain length implies that processing with dithiolsinfluences the physical interactions between the polymer chains and/orbetween the polymer and fullerene phases.

The novel methods described herein provide operationally simple andversatile tools for tailoring of the heterojunction solar cellmorphology in systems where thermal annealing is not effective. Thisapproach works even on a system in which polymer crystallinity is notobserved. Based on calculations by Brabec et. al. on photovoltaic cellsfabricated from PCPDTBT:C₇₁-PCBM, further optimization of morphology andequalization of ambipolar transport could lead to further increases inpower conversion efficiency. This expectation is fully consistent withthe Incident Photon Conversion Efficiency data shown in FIG. 37; thereis a clear opportunity to increase the IPCE.

The disclosures of all publications, patents, patent applications andpublished patent applications referred to herein by an identifyingcitation are hereby incorporated herein by reference in their entirety.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it is apparent to those skilled in the art that certainchanges and modifications will be practiced. Therefore, the descriptionand examples should not be construed as limiting the scope of theinvention.

What is claimed is:
 1. An electronic device comprising a firstelectrode, an organic semiconductor film having a first side and asecond side, where the first side of the organic semiconductor filmcontacts the first electrode and where the film is formed by adding anamount of one or more low molecular weight alkyl-containing molecules toa solution used to form the organic semiconductor film, and a secondelectrode contacting the second side of the organic semiconductor film,wherein the one or more low molecular weight alkyl-containing moleculesare selected from C₄-C₂₀ alkanes, C₄-C₁₆ alcohols, and C₄-C₁₆ thiols. 2.The device of claim 1, wherein the organic semiconductor film comprisesa conjugated polymer film.
 3. The device of claim 1, wherein theelectronic device is a solar cell, photovoltaic cell, photodetector,photodiode, or phototransistor.
 4. The device of claim 1, wherein thefirst electrode is a high work function material.
 5. The device of claim1, wherein the second electrode is a low work function material.
 6. Anelectronic device comprising a first electrode, an organic semiconductorfilm having a first side and a second side, where the first side of theorganic semiconductor film contacts the first electrode and where thefilm is formed by adding an amount of one or more low molecular weightalkyl-containing molecules to a solution used to form the organicsemiconductor film, and a second electrode contacting the second side ofthe organic semiconductor film, wherein the low molecular weightalkyl-containing molecules are selected from the group consisting ofC₁-C₂₀ alkanes substituted with one or more substituents selected fromaldehyde, dioxo, hydroxy, thiol, thioalkyl, amine, amide, thioether, andepoxide groups, with the proviso that if a thiol or hydroxy groupsubstituent is present, at least one independently chosen additionalsubstituent must also be present.
 7. The device of claim 6, wherein theorganic semiconductor film comprises a conjugated polymer film.
 8. Thedevice of claim 6, wherein the electronic device is a solar cell,photovoltaic cell, photodetector, photodiode, or phototransistor.
 9. Thedevice of claim 6, wherein the first electrode is a high work functionmaterial.
 10. The device of claim 6, wherein the second electrode is alow work function material.